1. Field of the Invention
The present invention relates to a conveyor-belt apparatus in which an endless belt circulates. Further, the present invention relates to an image heating apparatus in which the conveyor-belt apparatus may be applicable to, for example, a fixing device for heating a toner image in an image forming apparatus such as a copying machine or a printer.
2. Description of the Related Art
In general, in the image forming apparatus, a toner image formed on an image bearing member is transferred onto a sheet such as a recording material, and the sheet is conveyed to the fixing device in which the toner image is heated and pressed to be fixed onto the sheet, thereby recording and outputting the image. A typical fixing device is constructed of a roller pair including a heating roller and a pressure roller. A heater is installed inside the heating roller, and the pressure roller is brought into pressure-contact with the heating roller to form a fixing nip portion. The fixing nip portion nips the sheet to heat and apply pressure to the sheet.
By the way, in order to increase the image formation speed as much as possible, and in addition, to output a high definition, high quality image with a high glossiness, a method can be used involving the elongating of the period of time during which the sheet passes through the fixing nip portion as much as possible, thereby fusing the toner sufficiently to soften it. As a method for elongating the passing time period, the diameters of the heating roller and the pressure roller may be enlarged. However, in such a case, the fixing device is enlarged in size, resulting in a problem of enlargement in size of the image forming apparatus main body.
In order to elongate the sheet passing period of time, there is a recent tendency to employ a fixing device in which the fixing nip portion is formed with a belt type in place of a roller type which has been used before. Adoption of the belt type attains suppression of the size enlargement of the fixing device, and in addition, attains a high speed operation, thereby elongating the period of time during which the sheet passes. For example, Japanese Patent Application Laid-Open No. 2004-341346 describes an image forming apparatus and a fixing device, in which a necessary and sufficient nip width is secured in a sheet conveying direction. However, even in the belt type of apparatus, there is the following problem in spite of those advantages.
Either a heating belt or a pressure belt are an endless belt formed in an endless shape. The endless belt exhibits a peculiar behavior during circulation. Specifically, the belt moves alternately to right and left to be misaligned to perform serpentine movement in the belt width direction, which is orthogonal to a circulation direction, resulting in the dropping out of a belt driving roller, likely causing damage at right and left ends of the belt. A solution to these problems is important. Before this, the applicant of the present invention suggested a technology relating to prevention of the belt misalignment (see, for example, Japanese Patent Application Laid-Open No. H04-104180). This relates to a technology which includes a lateral movement control unit for causing a fixing film formed into an endless belt shape to an endless reciprocating motion within a predetermined range in the longitudinal direction, and in which a speed variable unit for varying a lateral speed is provided.
As described above, in the conveyor-belt apparatus in which a belt is passed over two roller members to be circulated, there is a case in which the belt laterally moves alternately to one side and the other side of its intended path to perform serpentine movement during the circulation. In that case, a so-called steering mechanism is operated to perform a return operation control such that the belt moves from the one side to the other side to thereby circulate the belt at a proper position. The steering mechanism refers to a technology in which, when the belt conducts the lateral movement, a rotation shaft of any one of the roller members is operated toward an inclined direction to return the belt which has been misaligned on the one side toward the other side, and this operation is repeated alternately to circulate the belt at the proper position.
However, if the durability of the belt is lowered or the belt is degraded due to use with time, there occurs a variation in friction pressure distribution due to a friction factor (μ) on an inner surface of the belt, resulting in a tendency for the belt to conduct the lateral movement toward a side where the friction pressure is lowered. This tendency of lateral movement becomes marked and the belt finally becomes out of control. The belt, being out of control, comes off from the roller members to drop out or to be damaged, and reaches the end of its life.
In order to prevent the belt from conducting lateral movement toward the side with the lower the friction factor, the steering amount can be increased so that an inclination angle of a roller rotating shaft is increased toward the steering direction. However, as the inclination angle of the roller rotating shaft is increased, there is a need to provide a space for accomodating the inclination. In such a case of the image forming apparatus, which is required to severly limit the space for installing components, it is particularly and significantly disadvantageous. For example, in a case of twin belt fuser (TBF), particularly, there is employed a two-axis steering mechanism using two roller members at an entry of an upstream side in a direction of conveying the recording sheet, and there is a limit to increase the steering amount because the increase of the steering amount affects the behavior of the recording paper.